As industry has turned to an extensive utilization of complex processing and control equipment, a concomitant need has arisen for specialized treatment of the environment within which this equipment operates. Exemplary of such equipment are computer installations which exhibit relatively high heat emission rates representing a generally continuous (year around) load to air conditioning equipment, the operational demands upon which have heretofore been typically seasonal in nature. In addition to requiring accurate removal of generated heat loads, such installations further require control over the relative humidity within their zonal environment.
Theretofore, the air conditioning industry has reponded to requirements for providing accurate temperature and humidity control within the enclosed regions of computer rooms and the like, on the one hand, with the somewhat brute force approach of operating typical refrigeration components to continuously treat air within the zone surrounding the equipment. As might be expected, the energy demands associated with such an approach do represent a significant cost factor. In another approach, advantage has been taken of seasonally lower outdoor (ambient) temperatures by a controlled mixing of cooler outdoor air with recirculated air. This commingling of air permits an intermittant deactivation of energy consuming refrigeration components.
Humidity control generally is carried out through the use of humidifiers to add moisture generally at the output region of air conditioning apparatus, while dehumidification, on the other hand, is a more elaborate process. Typically, dehumidification requires the operation of refrigeration components to remove moisture and this operation usually is associated with a reheating of the thus dehumidified but cooled air to regain the required zone target or "set point" temperature. One disadvantage attendant with the use of outside air to contribute to cooling stems from the general tendency of industry to locate computer rooms and the like well within the internal regions of buildings. As a consequence, access to outside air can be achieved only through the use of extensive and bulksome ducting, a requirement rendering the otherwise achieved energy conservation impractical and economically unsound. However, should the location disadvantage be overcome, typically encountered variations in ambient air humidity have been found to counteract the energy gains from utilizing outdoor air at lower temperature, inasmuch as a greater consumption of energy is required to carry out humidification and dehumidfication than initially conserved in using the outside air. This outside air also will contain undesirable contaminants such as dust and the like.
The first practical system introduced to industry wherein effective energy conservation was achieved taking advantage of lower outside or ambient air temperatures is described in U.S. Pat. No. 3,525,385. This system provides an outdoor heat exchanger which operates in conjunction with a liquid heat exchanging medium, such as glycol or the like, which is pumped through conduits into the building retaining the enclosed air conditioned region and at which point this fluid is utilized both to provide heat exchange within the condenser units of a refrigeration system and, alternately, to provide cooling through the use of a liquid cooling coil interposed within the air flow of the internally disposed air conditioning equipment. Thus, as outdoor temperatures drop below predetermined levels, i.e. about 35.degree. F., full advantage is taken of those lower levels to accommodate the heat loads imposed from computer equipment and the like. Alternately, in warmer seasons, the higher outdoor temperatures can be utilized to provide a reheat function utilizing the heat exchange liquid in conjunction with now operating refrigeration components. As industry has seen fit to adapt this energy conserving system, the term "free-cooling" has been found to be used to describe such an energy saving feature.
While the economies recognized with previous free cooling systems are important, their use is somewhat constrained in consequence of the number of days in a year wherein the temperature is adequately low, i.e. below 35.degree. F. Average temperature data are available for most American cities. An examination of these data reveals that in geographical areas currently comprising about 75% of industry and commerce, outdoor temperatures between 35.degree. F. and 65.degree. F. occur for at least of one-half of a given year, while temperatures of 35.degree. F. and below occur for less than 100 days of that year. Should a free cooling system be available taking advantage of the outdoor environment at temperatures above 35.degree. F., enhanced economies due to lower energy consumption will be realized through a more restrictive operation of mechanical refrigeration system components. In the latter regard, compressor operation alone is considered to account for about 70% of the total energy consumption of a mechanical refrigeration system.
Reference is made to U.S. Pat. Nos. 3,522,451; 3,714,980 and 3,844,475 which describe an arrangement wherein a temperature control actuator turns on and turns off multiple units in an environmental conditioning system as determined by need.